Sealing parts and gasket parts used in electrical appliances are essentially required to be excellent in barrier properties, sealing properties, etc., to satisfy desired hardness, to have high heat resistance so as to be able to withstand heat generated during application of electric current and to be employable also in low-temperature environment.
For example, for cell sealing members for fuel cells and the like, low-cost materials excellent in heat resistance, acid resistance, gas permeation resistance and high-speed moldability have been desired. In the existing circumstances, a fluororubber has been adopted from the viewpoints of heat resistance and acid resistance, a butyl rubber has been adopted from the viewpoint of gas permeation resistance, and a silicone rubber has been adopted from the viewpoints of heat resistance and moldability. However, when the high-speed moldability is required, usual materials are insufficient to meet the requirement, and studies have thus been made on, for example, a method in which a liquid silicone rubber is used and liquid injection molding (LIM) is applied. The silicone rubber is excellent in heat resistance and high-speed moldability, but it cannot be said that the silicone rubber is satisfactory in acid resistance and gas permeation resistance.
With minimization of size and increase in performance of electronic equipments, decrease in size and thickness of their components has been desired. However, when the size of the components is decreased, assembly workability in manufacturing is deteriorated. Therefore, integration and combination of various components have been desired.
For example, a gasket for a hard disk drive that is an electronic memory device is sometimes used in such a form that a simple rubber or a urethane foam sheet is interposed between metal covers such as stainless steel covers or aluminum covers and they are bonded with an adhesive and thereby integrated and then used. However, simultaneously with the integration, decrease in weight and thickness of the metal covers is carried out, and therefore, if the hardness (reaction force) of the gasket is high, a problem of deformation of the covers takes place.
Under such circumstances, a styrene-based thermoplastic elastomer has been disclosed for a gasket member (Patent document 1). This document describes that the styrene-based thermoplastic elastomer has low hardness and does not need a vulcanization process differently from rubber materials, which enables simplification of the production process and recycling of the elastomer.
However, the elastomer tends to be exposed to higher temperatures (particularly not lower than 80° C.) in the use environment because of generation of heat due to increase in performance (high rotational speed) of hard disk drives and adoption of the elastomer in automobiles. In such environment, the conventional styrene-based thermoplastic elastomer has a problem of permanent set at high temperatures, which is one of mechanical properties of the elastomer, and there is a limitation on the performance of the elastomer.
Electric wire connectors have functions of connecting and branching electric wires, and comprise a pair of male and female resin frames that can be connected to each other through one-touch operation, an electric wire and a sealing member. The sealing member is mainly used as a dust seal between the electric wire and the resin frame. The sealing member used in this type of the electric wire connector requires sealing properties and insertion properties for thin electric wires, and a silicone rubber and a nitrile rubber which have low hardness and which are oil-bleeding type rubbers have been used in the past. These rubbers, however, mainly contain silicone oil as a plasticizer, and the plasticizer adheres to electrical contact points during the use, resulting in occurrence of troubles in the flow of electric current due to insulation at the electrical contact points.
Under such circumstances, Patent document 2 has proposed a rubber composition, which is excellent in high-speed moldability, heat resistance, acid resistance and gas permeation resistance and which is favorably used for a fuel cell sealing member, a gasket member for an electronic device such as a hard disk drive, a sealing member for an electric wire connector, etc. This rubber composition is a liquid rubber composition which can be applied to LIM molding, and which can provide molded products of low hardness. Therefore, this rubber composition is suitable for a sealing member for a fuel cell, a gasket member for a hard disk drive, etc. for which sealing properties with low reaction force are required even under high compression. Such a rubber composition, however, is inferior to a silicone rubber and the like inelastic recovery rate in low-temperature environment, and therefore, further improvement in mechanical properties in low-temperature environment has been desired.    Patent document 1: Japanese Patent No. 2961068    Patent document 2: International Publication WO03/057777